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Effect of isolated roughness element height on high-speed laminar–turbulent transition

Published online by Cambridge University Press:  04 April 2017

David Estruch-Samper Open the ORCID record for David Estruch-Samper [Opens in a new window] ,
Richard Hillier ,
Leon Vanstone  and
Bharathram Ganapathisubramani
David Estruch-Samper*
Affiliation:
Department of Mechanical Engineering, National University of Singapore, 117575, Singapore
Richard Hillier
Affiliation:
Department of Aeronautics, Imperial College London, South Kensington, London SW7 2AZ, UK
Leon Vanstone
Affiliation:
Department of Aerospace Engineering, University of Texas at Austin, TX 78712, USA
Bharathram Ganapathisubramani
Affiliation:
Engineering and the Environment, Southampton University, Southampton SO17 1BJ, UK
*
†Email address for correspondence: mpedavid@nus.edu.sg
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Abstract

Understanding of the roughness-induced laminar–turbulent transition of supersonic and hypersonic flows is partly challenged by the intricate sensitivities presented by different correlation criteria. We investigate experimentally the effect of height for an isolated roughness element of quadrilateral planform. Heat transfer measurements document the enhancement of roughness-induced disturbances – here the associated heat flux perturbation – along a downstream axisymmetric laminar separation. With increasing element height $k$, a gradual intensification in wake disturbance levels is found for subcritical elements ($k/\unicode[STIX]{x1D6FF}_{k}<0.15$, where $\unicode[STIX]{x1D6FF}_{k}$ is the undisturbed boundary layer thickness) while elements taller than the effective condition ($k/\unicode[STIX]{x1D6FF}_{k}\geqslant 0.32$) bypass the more moderate transition mechanisms to produce a fully turbulent element wake. Results exhibit high sensitivity to flow properties at roughness height between critical and effective conditions. A reduction in wake disturbance levels with increasing height is documented within $0.23\leqslant k/\unicode[STIX]{x1D6FF}_{k}\leqslant 0.32$. This effect coincides with a decrease in kinematic viscosity at roughness height $\unicode[STIX]{x1D708}_{k}$ (as Mach number at height $M_{k}$ increases from 1.52 to 1.96) and is restricted to elements with strong local separation, whereby the influence of local shear effects is enhanced.

JFM classification

Compressible Flows: Compressible boundary layers Aerodynamics: High-speed flow Instability: Transition to turbulence
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 818 , 10 May 2017 , R1
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Copyright
© 2017 Cambridge University Press 

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